Fix races between tty.c and sessrele() / doenterpgrp() / leavepgrp(). The tty code is still under giant lock, but the session/pgrp release code just used proctree_locks. This explains why moving the proctree_lock in sys/kern/tty.c rev. 1.258 did fix the panics in our SMP systems.
Fix races between tty.c and sessrele() / doenterpgrp() / leavepgrp(). The tty
code is still under giant lock, but the session/pgrp release code just used
proctree_locks. This explains why moving the proctree_lock in sys/kern/tty.c
rev. 1.258 did fix the panics in our SMP systems.

This should also fix some race panics with revoked ttys.

Reviewed by:	jhb
MFC after:	1 week

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